How do you unseize a waverunner?

Take the head off. Spray the cylinders with PB Blaster and let sit for an hour and respray. Make a sturdy plug of wood to fit onto the pistons that will not get stuck at any point in the cylinder. Get a heavy mallet. Get a pipe wrench that will fit onto the crank sticking out of the engine going to the prop. Using the plug of wood hammer the pistons alternately while rotating the crank the best you can in the proper rotating direction. You can tell the direction by looking at the prop. You might just break it free. Before you start use some spray can computer duster to get the pistons cold by spraying with the can upside down and pour extremely hot boiling water into the cylinder jacket. That could help A LOT.